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Algebra Difficulty 6.1 National olympiad Prove it Balkan Mathematical Olympiad

Prove that there is a positive integer number nn such that the decimal representation of the number:
k=1n3(n3k)8k \sum_{k=1}^{\lfloor \frac{n}{3} \rfloor} \binom{n}{3k} 8^k
ends in 2023 digits 8.

Solution

Let f(n)=k=1n3(n3k)8kf(n) = \sum_{k=1}^{\lfloor \frac{n}{3} \rfloor} \binom{n}{3k} 8^k and ω1\omega \neq 1 be a third root of the unity. Using the fact that for every integer k0k \geq 0:
1+ωk+ω2k={3,if 3k0,otherwise, 1 + \omega^k + \omega^{2k} = \begin{cases} 3, & \text{if } 3 \mid k \\ 0, & \text{otherwise,} \end{cases}
we get that:
f(n)+1=k=0n3(n3k)2k=13k=0n(1+ωk+ω2k)(nk)2k=133n+13(1+2ω)n+13(1+2ω2)n. \begin{aligned} f(n) + 1 &= \sum_{k=0}^{\lfloor \frac{n}{3} \rfloor} \binom{n}{3k} 2^k = \frac{1}{3} \sum_{k=0}^{n} (1 + \omega^k + \omega^{2k}) \binom{n}{k} 2^k \\ &= \frac{1}{3} 3^n + \frac{1}{3} (1 + 2\omega)^n + \frac{1}{3} (1 + 2\omega^2)^n. \end{aligned}
Now note that 3,1+2ω3, 1+2\omega and 1+2ω21+2\omega^2 are the roots of the polynomial:
P(x)=(x3)(x12ω)(x12ω2)=(x1)38=x33x2+3x9 P(x) = (x-3)(x-1-2\omega)(x-1-2\omega^2) = (x-1)^3 - 8 = x^3 - 3x^2 + 3x - 9
which, in turn, is the characteristic polynomial of the recursive sequence (ai)i0(a_i)_{i \geq 0}:
ai+3=3ai+23ai+1+9ai for i0. a_{i+3} = 3a_{i+2} - 3a_{i+1} + 9a_i \text{ for } i \geq 0.
Thus, if we set ai=f(i)+1=1a_i = f(i) + 1 = 1 for 0i20 \le i \le 2, then f(n)+1=anf(n) + 1 = a_n for every n0n \ge 0. Let bi=ai(mod102023)b_i = a_i \pmod{10^{2023}}. Since gcd(3,102023)=1\text{gcd}(3, 10^{2023}) = 1, any three consecutive terms of the sequence (bi)i0(b_i)_{i \ge 0} uniquely determine the previous as well as the next term of this sequence. Together with the fact that there are only finitely many residues modulo 10202310^{2023}, we conclude that the sequence (bi)i0(b_i)_{i \ge 0} is periodic with some period d>3d > 3 (since b3=a3=9b_3 = a_3 = 9). Therefore:
9(f(d1)+1)=9ad1=ad+23ad+1+3ada23a1+3a0(mod102023)=1(mod102023). 9(f(d-1)+1) = 9a_{d-1} = a_{d+2}-3a_{d+1}+3a_d \equiv a_2-3a_1+3a_0 \pmod{10^{2023}} = 1 \pmod{10^{2023}}.
Finally, since 98.102023+19 \mid 8.10^{2023} + 1, we conclude that 98.102023+191(mod102023)9^{\frac{8.10^{2023}+1}{9}} \equiv 1 \pmod{10^{2023}} and consequently:
f(d1)+1=ad18.102023+19=88892022(mod102023) f(d-1) + 1 = a_{d-1} \equiv \frac{8.10^{2023} + 1}{9} = \underbrace{88 \dots 89}_{2022} \pmod{10^{2023}}

and thus f(d1)8882022(mod 102023)f(d-1) \equiv \underbrace{88 \dots 8}_{2022} (\text{mod } 10^{2023}). Therefore n=d1n = d-1 has the desired property. \square

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